The assignee of the present invention manufactures and deploys spacecraft for, inter alia, communications, broadcast and imaging services, more particularly, spacecraft having a nadir facing payload and capable of generating a substantial amount of DC power. One class of orbits of interest for such spacecraft includes Low Earth Orbit (LEO), the term LEO generally being considered to relate to orbits having an altitude between 160 and 2000 km.
Spacecraft in such orbits experience disturbance torques and drag from aerodynamic forces resulting from impingement of atmospheric gas molecules, atoms or other particles on spacecraft surfaces. Aerodynamic forces diminish rapidly as a function of increasing orbital altitude, and are generally no greater than solar radiation forces at altitudes above 800 km. As a result, some spacecraft operators avoid LEO orbits with altitudes below about 800 km. For example, Globalstar spacecraft operate at an altitude of about 1,400 km; as a further example, Iridium spacecraft operate at an altitude of about 780 km. Where spacecraft operating in LEO orbit are required to operate at orbits having an altitude less than 600 km (e.g., the International Space Station at about 400 km) a very appreciable amount of propellant is ordinarily required to offset aerodynamically induced drag forces and torques, so as to maintain a desired orbital altitude and attitude.
Thus, improved techniques for maintaining a desired orbital altitude and attitude are desirable for spacecraft operating in atmospheric conditions typical of LEO orbits having an altitude in the range of 200 to 800 km are desirable.